Connected: An Internet Encyclopedia
RFC 1583 - 12.1.4. Link State ID

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### 12.1.4. Link State ID

This field identifies the piece of the routing domain that is being described by the advertisement. Depending on the advertisement's LS type, the Link State ID takes on the values listed in Table 16.

Actually, for Type 3 summary link (LS type = 3) advertisements and AS external link (LS type = 5) advertisements, the Link State ID may additionally have one or more of the destination network's "host" bits set. For example, when originating an AS external link for the network 10.0.0.0 with mask of 255.0.0.0, the Link State ID can be set to anything in the range 10.0.0.0 through 10.255.255.255 inclusive (although 10.0.0.0 should be used whenever possible). The freedom to set certain host bits allows a router to originate separate advertisements for two networks having the same address but different masks. See Appendix F for details.

               LS Type   Advertisement description
               __________________________________________________
               1         These are the router links
                         advertisements. They describe the
                         collected states of the router's
                         interfaces. For more information,
                         consult Section 12.4.1.
               __________________________________________________
               2         These are the network links
                         advertisements. They describe the set
                         of routers attached to the network. For
                         more information, consult
                         Section 12.4.2.
               __________________________________________________
               3 or 4    These are the summary link
                         advertisements. They describe
                         inter-area routes, and enable the
                         condensation of routing information at
                         area borders. Originated by area border
                         routers, the Type 3 advertisements
                         describe routes to networks while the
                         Type 4 advertisements describe routes to
                         AS boundary routers.
               __________________________________________________
               5         These are the AS external link
                         advertisements. Originated by AS
                         boundary routers, they describe routes
                         to destinations external to the
                         Autonomous System. A default route for
                         the Autonomous System can also be
                         described by an AS external link
                         advertisement.

                   Table 15: OSPF link state advertisements.

                LS Type   Link State ID
                _______________________________________________
                1         The originating router's Router ID.
                2         The IP interface address of the
                          network's Designated Router.
                3         The destination network's IP address.
                4         The Router ID of the described AS
                          boundary router.
                5         The destination network's IP address.

                  Table 16: The advertisement's Link State ID.

When the link state advertisement is describing a network (LS type = 2, 3 or 5), the network's IP address is easily derived by masking the Link State ID with the network/subnet mask contained in the body of the link state advertisement. When the link state advertisement is describing a router (LS type = 1 or 4), the Link State ID is always the described router's OSPF Router ID.

When an AS external advertisement (LS Type = 5) is describing a default route, its Link State ID is set to DefaultDestination (0.0.0.0).

Next: 12.1.5. Advertising Router

Connected: An Internet Encyclopedia
RFC 1583 - 12.1.4. Link State ID